1. Field of the Invention
The present invention relates to color processing that performs color conversion based on a color appearance model.
2. Description of the Related Art
The Commission Internationale de L'eclairage (CIE) specifies a color appearance model (CAM) which converts tristimulus values, i.e., XYZ values of a certain color into attribute values (perceptual lightness J value, perceptual chroma C value, and perceptual hue h value) that represent color appearance. For example, CIECAM02 specified by reference 1 (CIE TECHNICAL REPORT “A COLOR APPEARANCE MODEL FOR COLOR MANAGEMENT SYSTEMS: CIECAM02” CIE159: 2004, ISBN3901906290) models a visual phenomenon such as adaptation to illuminating light using mathematical expressions. According to CIECAM02, parameters indicating viewing conditions of an image on the input side of the image, and those indicating viewing conditions of the image on the reproduction side of the image are configured. When a color value on the input side is forward converted into that on a color appearance space, and the converted value is then inversely converted into a color value on the reproduction side, the color appearance in a viewing environment on the input side can match that in a viewing environment on the reproduction side. For this reason, the CAM is popularly used in a color management system (CMS) that consolidates color spaces and color reproduction characteristics of a plurality of different devices.
In recent years, the CIE has proposed an image adaptive color appearance model (iCAM: image color appearance model). iCAM enhances color processing of a color appearance model (CAM) in consideration of local color adaptation processing and frequency characteristics of the vision. The local color adaptation processing of iCAM defines a visual field of a viewer, calculates viewing condition parameters as conversion parameters of the CAM based on pixel data distributed in the visual field, and executes color conversion by applying the viewing condition parameters to respective pixels of an image.
iCAM can express lightness levels and colors perceived by a human as numerical values, and is used in, for example, a technique for converting the appearance of lightness levels and colors perceived by a human in a photographing scene into those that can be reproduced by an image output device. More specifically, iCAM is used in a dynamic range compression technique required to preferably reproduce a high-dynamic-range (HDR) image. For example, reference 2 (Japanese Patent Laid-Open No. 2006-313465) proposes tonality compression processing which applies a technique of iCAM06 that adopts the concept of local adaptation processing (see Kuang, J., Johnson, G. M., Fairchild M. D. “iCAM06: A refined image appearance model for HDR image rendering” Journal of Visual Communication, 2007).
However, upon application of the local adaptation processing, some images may cause failures in reproduction of colors and tones. When an image of a person is reproduced using iCAM, if an object having a high-chroma color exists around a face-image area, the color of a face image becomes unnatural. For example, when a red object exists near a face image, the flesh color of the face image is reproduced to be greenish. When a cyan object exists, the flesh color is reproduced to be magenta-tinged. In other words, even in images which are obtained by photographing an identical person and have an identical flesh color, consistency of flesh color reproduction cannot be obtained depending on objects included in the images, and preferred image reproduction cannot often be obtained.
iCAM determines a white point (adapting white point) as one of the conversion parameters from pixel values in a surrounding area of image data of interest. Therefore, an adapting white point of a solid area (flat part) in an image is also determined from pixel values in a surrounding area, and processing is applied to the solid area based on different adapting white points. As a result, the solid area cannot be preferably reproduced.